You are very welcome!
This is the very reason why I reported it here - to save others the nightmare I went through when plasma stopped working without any hint in the logs what the reason could be (I thought about firwmare/drivers, xorg, etc packages which updated at the same time - but it was neither of these!).

I was able to actually determine the core of the issue only after I gave up KDE/plasma and installed other DE (xfce) - and then noticing not only kf5 apps were not working there; but also apps like vlc, exo-launch etc.

Hello All, and manul
Yes Plasma went away after a D-U just the other day - I saw your message and went looking for its cause - I noticed that I had libqt5xcbqpa5 but no Plasma - I tried to apt-get to reinstall but something else seemed wrong with what it wanted to remove if I was to reinstall it. just remove permenently...
Lots of things kde5 related. So after a day, I got brave again and went to apt-get after apt update was completed.
I typed in apt-get install kde-full and a lot of applications wanted to both get removed and installed, all seemingly related to KDE.
only 4 new applications other that the large list and nothing about removal without a replacement. it was a matching list.
so i did it - after a complete reboot KDE plasma was back and at this time all seems as it was before.
Plasma 5 is back... don't quite know what it meant but there it is.
all works as far as I can see.

A good start would be to look into the changes introduced by the last d-u you did (the one after which plasma stopped working): which packages have been installed/upgraded/removed? (you can check this in apt logs)

plasma-desktop was not installed.
Now I can get into KDE but i think it is misbehave compared with 386 installation. Probably many KDE4 left behind.
Is there a way to check and remove unnecessary kde components?

plasma-desktop was not installed.
Now I can get into KDE but i think it is misbehave compared with 386 installation. Probably many KDE4 left behind.
Is there a way to check and remove unnecessary kde components?

First you may want to assure you have the following lines into your some of your apt conf files (I have these in /etc/apt/apt.conf.d/80no-recommends-suggests )

APT::Get::AutomaticRemove "0";

and assure if such line exists it is commented (or set to 0)
#// APT::Get::HideAutoRemove "1";

This way apt will let you know which packages can be removed (but not remove them automatically - i.e. you still have to manually run "apt-get autoremove" to clear these).